The present invention relates in general to a laser device, and more particularly, to a wavelength tunable laser device.
In recent years, there has been a dramatic increase in demand for communication systems involving communication networks, such as dense wavelength division multiplexing (DWDM) networks, dynamic routing and packet switching networks, Coarse wavelength division multiplexing (CWDM) networks, and analog photonics. These communication networks are generally based on optical technologies using optical fiber as a communicating medium and therefore known as optical communication networks. In optical communication networks, data is communicated or transmitted through an optical fiber medium as laser beams at different wavelengths. The wavelength of the laser beams that is used to transmit data can be selected from a wide range of wavelengths for the optical communication networks. For such networks, a wavelength tunable laser device is desired that can select and tune a laser beam to a desired wavelength among the wide range of wavelengths.
Currently, various laser devices capable of providing a wavelength tunable laser are available. One such laser device is the Sampled-Grating DBR (SGDBR) laser. The SGDBR laser includes monolithic integration with a front and back Bragg grating, known as distributed Bragg reflector (DBR), a phase section, and a gain section. In this laser, currents are injected into the front and back grating sections for wavelength tuning. The phase section is used to control the phase of the laser by using the error signal that is generated using the correlation between the output power and the cavity-mode frequency offset. The phase control in the SGDBR is required to reduce the cavity-mode frequency offset to zero. Another laser device used for wavelength tuning is a microelectromechanical tunable external cavity laser (MEM-ECL). In this laser, a microelectromechanical (MEM) device is used to tune a mirror in front of an intra-cavity diffraction grating for wavelength tuning. The phase control of the laser beam is performed by using a piezoelectric transducer (PZT) on the mirror, using the error signal that is generated utilizing the correlation between the output power and the cavity-mode frequency offset. Apart from SGDBR and MEM-ECL, the tunable filter external cavity laser (TF-ECL) is also used for wavelength tuning. The TF-ECL uses an intra-cavity tunable filter for wavelength tuning. In TF-ECL, similar to MEM-ECL, the phase control of the laser beam is performed by using a PZT on the mirror.
All of the laser devices described above has their shortcomings. In the case of the SGDBR laser, the power consumption of the laser module is high. The currents required for the front and back grating sections are typically around 150 mA each. In addition, 100 mA of current is required for the gain section, resulting in a high operating current of 500 mA (approx.) The power consumption of the laser module is also increased by the use of a thermoelectric cooler (TEC) to cool the chip in a packaged module. As a result, high power (approx. 10 W) is required to feed the laser module. Therefore, the SGDBR is a power-consuming laser device and not economical.
Another shortcoming of the SGDBR laser is its difficulty to detect the cavity-mode frequency offset, which is required for the phase control of the laser, in order to optimize the performance of the laser. The phase of the laser beam is controlled by using the correlation between the cavity-mode frequency offset and the laser output power. The cavity-mode frequency offset falls to zero when the power/voltage is at a local maximal/minimal. Therefore, to lock the cavity-mode frequency offset at zero, the SGDBR laser involves dithering the gain section's injection current and using the phase-sensitive lock-in technique to lock to local power peak/dip by feeding back to the phase section. This phase-sensitive lock-in technique requires two lock-in amplifiers and various feedback loops controlled by an on-board microprocessor. The overall structure therefore results in a complex laser control circuit and increases the cost of SGDBR.
The MEM-ECL laser also includes the complex control circuit to control the MEM mirror tuning and the phase control of the laser. The packaging cost of the MEM-ECL is also high due to the alignment requirement of micro-optic components. The TF-ECL laser eliminates the complex circuitry required for wavelength tuning but still requires a complex control circuit for phase section control. Furthermore, TF-ECL generally takes around 25 seconds for wavelength tuning, which is longer than that of the MEM-ECL and SGDBR laser devices.
In light of the drawbacks of the known laser devices mentioned above, there is a need for a wavelength tunable laser that requires low operating current and power, and has a simple control circuit for phase control and wavelength tuning. The laser device should also have a high wavelength tuning speed.